Vortex shedding and drag force reduction

ABSTRACT

Smoothly curved protuberances ( 8, 26, 34 ) are added to an elongate body ( 2, 16, 18, 20, 22, 32 ) to modify the fluid flow to reduce the drag Force F drag  and the forces induced by vortex shedding F vortex . The protuberances can be arranged in diametrically opposed pairs with longitudinally adjacent pairs having different radial directions first to cope with fluid flow from a variety of different directions.

[0001] This invention relates to techniques for modifying fluid flow soas to reduce the effects of drag and vortex shedding. More particularly,this invention relates to such techniques that may be applied toelongate bodies.

[0002] When an elongate body, such as a chimney, is positioned within anenvironment where it is subject to fluid flow, in the case of a chimneyairflow, then a drag force is exerted on the elongate body and vortexshedding can occur inducing forces that can lead to undesirablevibration. The drag force of passing fluid flow often means that theelongate body has to be produced with a strengthened structure to resistsuch a drag force. The cost of strengthening the structure in this waycan be significant. In the case of vortex shedding, the forces thisexerts vary with time in a manner that can establish highly damagingundesirable vibrations within an elongate body. It may be that thesevibrations will stimulate a resonance with potentially destructiveconsequences.

[0003] It is known to fit fairings to structures in order to modifyfluid flow around those structures to reduce drag. A problem with suchfairings is that they are usually only able to cope with fluid flow froma single direction and if the fluid flow direction changes, then theymay be ineffective, or in fact increase drag. The fairings may be mademovable to accommodate different flow directions, but thisdisadvantageously increases their cost and complexity.

[0004] It is also known to attach structures to elongate bodies in anattempt to reduce vortex shedding. An example of this is a helicalstrake that can be applied to the outside of a chimney. Whilst such ahelical strake may reduce vortex shedding, it often has the effect ofincreasing drag with a disadvantageous need to increase the strength ofthe chimney. An alternative is the use of a perforated shroud over achimney. Such perforated shrouds have been found to be too expensive tobe practical.

[0005] U.S. Pat. No. 4,059,129 discloses a cylindrical body with aplurality of small planes extending from its outer surface acting asvortex generators to reduce vibrations due to transverse flow againstthe body.

[0006] Discussions of vortex shedding may be found in E. Naudascher, D.Rockwell “FLOW-INDUCED VIBRATIONS an Engineering Guide”, IAHR-AIRH,Hydraulic structures design manual, A. A,Balkema/Rotterdam/Brookfield/1994, 160-176 and M. M. Zdravkovich,“Review and Classification of Various Aerodynamic and Hydrodynamic Meansfor Suppressing Vortex Shedding,” Journal of Wind Engineering andIndustrial Aerodynamics, 7 (1981) 145-189.

[0007] A description of a unidirectional fairing for use on a drillingriser to reduce vortex induced vibration is described in United StatesPatent U.S. Pat. No. 6,048,136.

[0008] Viewed from one aspect the present invention provides an elongatebody (2) having a plurality of longitudinally spaced apart smoothlycurved protuberances (8) extending therefrom, said protuberances havinga surface which is smoothly curved in two orthogonal directions andbeing shaped and dimensioned to modify fluid flow around said elongatebody in a manner that reduces forces upon said elongate body produced bydrag and vortex shedding.

[0009] The invention recognises and exploits the phenomenon whereby asmoothly curved protuberance (smooth at least in the sense of how itmodifies the fluid flow) from an elongate body can be made to modify thefluid flow around that body in a manner that reduces the forces exertedon the body by drag and vortex shedding. The protuberance isadvantageously simple and inexpensive to provide with or add to anelongate body.

[0010] Whilst the invention could be used in situations where the fluidflow was unidirectional, in preferred embodiments of the invention saidprotuberances extend in a plurality of different radial directions froma longitudinal axis of said elongate body.

[0011] This feature of the invention allows fixed protuberances that areinexpensive and simple to reduce drag and vortex shedding that can occurfrom fluid flow incident from any radial direction around the elongatebody. This is strongly advantageous since, for example, a chimney or adrilling platform leg may be subject to fluid flow from any radialdirection.

[0012] It will be appreciated that the differences between the radialdirection of adjacent protuberances may vary over a range of values. Ithas been found that a preferred range of values for the differencesbetween radial directions is 30 degrees to 90 degrees inclusive. Aparticularly preferred arrangement that works well in many cases is whenthe difference in radial direction between adjacent protuberances issubstantially 45 degrees.

[0013] The protuberances could be applied to a single side of theelongate body. However, in preferred embodiments the protuberances arearranged in pairs at the same longitudinal position along the elongatebody and with opposite radial directions. This has been found to beconstructionally convenient and provide good omni-directionalperformance.

[0014] The size of the protuberances can vary significantly dependingupon the circumstances. Generally speaking, more dense fluids mayrequire more pronounced protuberances than less dense fluids. It willalso be appreciated that the protuberances should not be too large orthey may result in an undesirably large increase in drag when the fluidflow is not favourably aligned with them.

[0015] Compared with the maximum diameter D of the cross-section of theelongate body, a preferred range of protuberance sizes has been found tobe one in which the protuberances extend from an outer surface of theelongate body by a distance within the range 0.1D to 0.75D. A morehighly preferred range is 0.25D to 0.5D.

[0016] The longitudinal spacing of the protuberances can also vary.Placing the protuberances too close together will increase cost andweight whilst it may also reduce the effectiveness of the protuberancesin modifying the fluid flow in the desired manner. Similarly, placingthe protuberances too far apart will make them ineffective. In preferredembodiments of the invention the longitudinal spacing of theprotuberances is such that said radial directions of said protuberancesvary along said longitudinal axis in a repeating pattern with a repeatdistance within the range 3D to 9D inclusive.

[0017] It will be appreciated that the smoothly curved protuberancescould have a wide variety of cross-sectional shapes. The protuberancesshould be smoothly curved and blend well into the shape of the rest ofthe elongate body so as to reduce drag. has been found to produce goodresults is when the cross-sectional shape of the protuberances is atleast a potion of an ellipse. When the protuberances are pairedtogether, they may be arranged in a fashion in which the back-to-backprotuberances have a combined cross-sectional shape that is a fullellipse.

[0018] The elongate body to which the protuberances are attached couldsimilarly have a range of cross-sectional shapes. However, a circularcross-sectional shape is common in bodies that are subject to the dragand vortex shedding forces which the invention seeks to reduce and thisshape has been found to benefit well from the technique of the presentinvention.

[0019] The elongate body around which the fluid flow is modified by thetechnique of the present invention could be part of a wide variety ofdifferent structures. Examples of structures that may particularlybenefit from the technique of the invention are an offshore riser, asupport member of an offshore platform, a pipe, an underwater cable,chimney and a support tower for a wind turbine.

[0020] It will be appreciated that the fluid which gives rise to thedrag and vortex shedding may be either a liquid or a gas.

[0021] The protuberances could be integrally formed with the elongatebody with which they are associated. However, in preferred embodimentsof the invention the protuberances may take the form of fairings (e.g.an element added to modify fluid flow) that are attached to an elongatebody. The engineering of many elongate bodies is in many cases alreadyfixed and the form of the invention as add-on fairings is particularlyconvenient and simple together with allowing the possibility forretro-fitting.

[0022] Viewed from another aspect the present invention provides amethod of reducing fluid flow induced forces upon an elongate bodyproduced by drag and vortex shedding, said method comprising the step ofproviding a plurality of fluid flow modifying longitudinally spacedapart smoothly curved protuberances extending from said elongate body,said protuberances having a surface which is smoothly curved in twoorthogonal directions.

[0023] Viewed from a further aspect the present invention provides a kitfor modifying fluid flow around an elongate body, said kit comprising aplurality of smoothly curved firings for fixing to said elongate bodyand a plurality of fairing fasteners for fixing said fairings to saidelongate body to form a plurality of longitudinally spaced apartsmoothly curved protuberances extending therefrom, said protuberanceshaving a surface which is smoothly curved in two orthogonal directionsand being shaped and dimensioned to modify fluid flow around saidelongate body in a manner that reduces forces upon said elongate bodyproduced by drag and vortex shedding.

[0024] Supplying the fairings and associated fasteners as a kit is alikely way in which the invention may be embodied in circumstances whenit is desired to retro-fit existing structures.

[0025] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

[0026]FIG. 1 schematically illustrates fluid flow past a circularcross-section body with associated drag and vortex shedding;

[0027]FIG. 2 illustrates a cylindrical elongate body having flowmodifying protuberances attached thereto;

[0028]FIG. 3 illustrates cross-sectional views of a pair of flowmodifying protuberances;

[0029]FIG. 4 illustrates a range of protuberances sizes applied to acylindrical body;

[0030]FIG. 5 illustrates a kit form of the protuberances; and

[0031]FIGS. 6, 7 and 8 illustrate possible uses of the invention.

[0032]FIG. 1 schematically illustrates a cylindrical body 2 positionedwithin a fluid flow 4. The fluid flow 4 gives rise to a drag forceF_(drag) acting in the same direction as the fluid flow 4. Vortices 6are shed from alternating sides of the cylinder 2 and moved downstreamwithin the fluid flow 4. As these vortices 6 are shed, they subject thecylinder 2 to a varying vortex shedding force F_(vortex) that is of agenerally periodic nature. The vortex shedding force F_(vortex) can varyin magnitude, direction and timing.

[0033] The drag force F_(drag) can necessitate an undesirable need toincrease the structural strength of the cylinder 2. The vortex sheddingforce F_(vortex) can similarly require the structure of the cylinder tobe strengthened as well as raising the possibility of inducingundesirable vibrations, or even resonance, within the cylinder 2.

[0034]FIG. 2 illustrates an elongate body in the form of a cylinder towhich smoothly curved protuberances have been added. These protuberancesare arranged in diametrically opposed pairs with the radial direction ofthe protuberances varying by substantially 45 degrees between adjacentpairs of protuberances. In a test the arrangement illustrated in FIG. 2produced a 24% drag reduction compared with the plain cylinder and alsoled to significantly less vortex-induced vibration.

[0035] In the specific example illustrated in FIG. 2, the protuberanceshave an elliptical cross-section and protrude by 0.5D from the surfaceof the cylinder where D is the diameter of the cylinder. Theprotuberances are spaced at an interval of 1.75D along the length of thecylinder in an arrangement where the orientation of the protuberancesrepeats at a distance of 7D.

[0036]FIG. 3 schematically illustrates cross-sectional views through apair of protuberances as illustrated in FIG. 2. The end view shows theelliptical form of the protuberances. In the illustrated example, themajor axis of the ellipse is W in length and the minor axis of theellipse is D in length corresponding to the diameter of the cylinder Don which the protuberance is mounted. The plan view shows theprotuberances to have a plan cross-section that is part of a circle ofdiameter W.

1. An elongate body (2) having a plurality of longitudinally spacedapart smoothly curved protuberances (8) extending therefrom, saidprotuberances having a surface which is smoothly curved in twoorthogonal directions and being shaped and dimensioned to modify fluidflow around said elongate body in a manner that reduces forces upon saidelongate body produced by drag and vortex shedding.
 2. An elongate bodyas claimed in claim 1, wherein said protuberances extend in a pluralityof different radial directions from a longitudinal axis of said elongatebody.
 3. An elongate body as claimed in claim 2, wherein longitudinallyadjacent protuberances have radial directions differing by an anglewithin the range 30 to 90 degrees inclusive.
 4. An elongate body asclaimed in claim 3, wherein longitudinally adjacent protuberances haveradial directions differing by an angle of substantially 45 degrees. 5.An elongate body as claimed in any one of the preceding claims, whereinat least two of said protuberances are arranged as a pair ofprotuberances having a common longitudinal position along said elongatebody and extending in opposite radial directions from said elongatebody.
 6. An elongate body as clamed in claim 5, wherein all of saidprotuberances are arranged as pairs of protuberances.
 7. An elongatebody as claimed in any one of the preceding claims having a maximumcross-sectional diameter excluding said protuberances of D.
 8. Anelongate body as claimed in claim 7, wherein said protuberances extendfrom an outer surface of said elongate body excluding said protuberancesby a distance within the range 0.1D to 0.75D inclusive.
 9. An elongatebody as claimed in claim 8, wherein said protuberances extend from anouter surface of said elongate body excluding said protuberances by adistance within the range 0.25D to 0.5D inclusive.
 10. An elongate bodyas claimed in claims 3 and 7, wherein said radial directions of saidprotuberances vary along said longitudinal axis in a repeating patternwith a repeat distance within the range 3D to 9D inclusive.
 11. Anelongate body as claimed in any one of the preceding claims, whereinsaid protuberances have a cross-sectional shape substantiallycorresponding to at least a portion of an ellipse.
 12. An elongate bodyas claimed in any one of the preceding claims, wherein said elongatebody has a cross-sectional shape substantially corresponding to acircle.
 13. An elongate body as claimed in any one of the precedingclaims, wherein said elongate body is one of: an offshore riser (20); asupport member of an offshore platform (16); a pipe an underwater cable;a chimney (22); and a support tower (32) for a wind turbine.
 14. Anelongate body as claimed in any one of claims 1 to 12, wherein saidfluid flow is liquid flow.
 15. An elongate body as claimed in any one ofclaims 1 to 12, wherein said fluid flow is gas flow.
 16. An elongatebody as claimed in any one of the preceding claims, wherein saidprotuberances are formed as fairings fixed to said elongate body.
 17. Amethod of reducing fluid flow induced forces upon an elongate bodyproduced by drag and vortex shedding, said method comprising the step ofproviding a plurality of fluid flow modifying longitudinally spacedapart smoothly curved protuberances extending from said elongate body,said protuberances having a surface which is smoothly curved in twoorthogonal directions.
 18. A kit for modifying fluid flow around anelongate body, said kit comprising a plurality of smoothly curvedfairings for fixing to said elongate body and a plurality of fairingfasteners for fixing said fairings to said elongate body to form aplurality of longitudinally spaced apart smoothly curved protuberancesextending therefrom, said protuberances having a surface which issmoothly curved in two orthogonal directions and being shaped anddimensioned to modify fluid flow around said elongate body in a mannerthat reduces forces upon said elongate body produced by drag and vortexshedding.